Load cells with overload protection and moment adjustment means

Abstract

A load cell includes base and load receiving members connected together by a pair of parallel flexures to form a parallelogram configuration. One of the flexures has at least one strain gage secured thereon, along with a cantilevered stop member positioned between the base and load receiving members. The stop member is suspended between the ends of a pair of adjustable set screws threadably mounted in the base and load receiving members to provide for an overload and underload protection of the load cell. Additional adjustable set screws are strategically placed in the load cell to further provide for the selective adjustment and redistribution of stresses sensed by the strain gage whereby the same electrical signal will be produced independent of the location of a load along a longitudinal axis of the load cell or a second axis perpendicular thereto. Thus, the load cell cannot be overstressed and can be adjusted to provide the same weight readout regardless of the position of the load on a platter secured on the load receiving member.

I claim: 
     
       1. A load cell comprising a base member adapted for securance to a stationary base,   a load receiving member spaced apart from said base member and adapted to have a load imposed thereon,   flexure means secured between said base and said load receiving members for flexing in response to said load, said flexure means comprising first and second flexures spaced apart in parallel relationship relative to each other and secured between said base and load receiving members to form a parallelogram therewith,   sensing means for producing a variable signal in response to the magnitude of said load, and   load protection means for delimiting flexing of said load receiving member and said flexure means in a first direction when the load imposed on said load receiving member exceeds a predetermined maximum, said load protection means comprising a stop member secured in cantilevered relationship on said first flexure and disposed between said base and load receiving members and stop means adjustably mounted on said base member for engaging said stop member when the load imposed on said load receiving member exceeds said predetermined maximum.   
     
     
       2. The load cell of claim 1 wherein said load protection means further delimits flexing of said load receiving member and said flexure means in a second direction, opposite to said first direction, when a second load imposed on said load receiving member exceeds a second predetermined maximum. 
     
     
       3. The load cell of claim 1 wherein said stop means comprises a set screw threadably mounted in said base member and having an end thereof positioned closely adjacent to said stop member to define a clearance therebetween. 
     
     
       4. The load cell of claim 1 wherein said base and load receiving members and said flexures are all formed from a unitary block of material and wherein each of said first and second flexures comprises a pair of longitudinally spaced flexure connections each formed by at least one arcuate cutout, said sensing means comprising at least one strain gage secured on said first flexure between the flexure connections thereof. 
     
     
       5. The load cell of claim 4 wherein a first flexure connection of said first flexure is vertically aligned with a first flexure connection of said second flexure and a second flexure connection of said first flexure is vertically aligned with a second flexure connection of said second flexure. 
     
     
       6. The load cell of claim 1 wherein said load protection means further delimits flexing of said load receiving member and said first and second flexures in a second direction, opposite to said first direction, when a second load imposed on said load receiving member exceeds a second predetermined maximum. 
     
     
       7. The load cell of claim 6 wherein said stop means comprises a set screw threadably mounted in said base member and having an end thereof positioned closely adjacent to said stop member to define a clearance therebetween and wherein said load protection means further comprises a second stop means adjustably mounted on said load receiving member for engaging said stop member when said second load exceeds said second predetermined maximum. 
     
     
       8. The load cell of claim 7 wherein said second stop means comprises a second set screw threadably mounted in said load receiving member and having an end thereof positioned closely adjacent to said stop member to define a clearance therebetween. 
     
     
       9. A load cell comprising a base member adapted for securance to a stationary base,   a load receiving member spaced apart from said base member and adapted to have a load imposed thereon   flexure means secured between said base and load receiving members for flexing in response to said load,   sensing means for producing a signal in response to stresses sensed thereby due to the magnitude of said load,   adjustment means for selectively redistributing said stresses to provide at least substantially the same said signal independent of the location of said load on said load receiving member, said flexure means comprising at least one flexure secured between said base and load receiving members and wherein said adjustment means comprises first adjustment means interconnected between said flexure and at least one of said base and load receiving members for selectively moving said flexure relative thereto, said first adjustment means comprising a cutout defined between said flexure and said one member, a set screw sequentially extending through said first flexure in unattached relationship therewith and through said cutout into threaded engagement in said one member, a head of said set screw being disposed on an outer side of said flexure.   
     
     
       10. The load cell of claim 9 wherein said first adjustment means further comprises a nut threadably mounted on said set screw and disposed within said cutout for selective bearing engagement with an inner surface of said first flexure whereby torquing-down of said set screw to engage said head with the outer surface of said flexure will place a shank portion of said set screw in tension and torquing-down of said nut on said set screw and into bearing engagement with the inner surface of said flexure will place a shank portion of said shank portion in compression. 
     
     
       11. The load cell of claim 9 wherein said sensing means comprises at least one strain gage secured on said flexure. 
     
     
       12. The load cell of claim 9 wherein said flexure means further comprises a second flexure spaced apart in parallel relationship relative to said first-mentioned flexure and secured between said base and load receiving members to form a parallelogram therewith and wherein said first adjustment means is interconnected between said first-mentioned flexure and said base member for selectively redistributing stresses in said first-mentioned flexure and said adjustment means further comprises second adjustment means interconnected between said second flexure and each of said base and load receiving members for selectively redistributing stresses in said second flexure. 
     
     
       13. The load cell of claim 12 wherein said second adjustment means comprises a first cutout defined between a first end of said second flexure and said base member, a second cutout defined between a second end of said second flexure and said load receiving member and first and second set screws extending in unattached relationship through each of the first and second ends of said second flexure, respectively, and threadably attached to a respective one of said base and load receiving members. 
     
     
       14. The load cell of claim 13 wherein each said set screw comprises a head mounted on an outer side of said second flexure and a nut threadably mounted on each said set screw and disposed within a respective one of said first and second cutouts to engage an innersurface of said flexure in bearing relationship therewith. 
     
     
       15. The load cell of claim 14 wherein said first adjustment means comprises a third cutout defined between an end of said first-mentioned flexure and said base member, a third set screw extending in unattached relationship through said first mentioned flexure and threadably attached to said base member and a nut threadably mounted on said set screw and disposed within said cutout, a head of said third set screw disposed on an outer side of said first-mentioned flexure. 
     
     
       16. A load cell comprising a base member adapted to securance to a stationary base,   a load receiving member spaced apart from said base member and adapted to have a load imposed thereon,   flexure means secured between said base and load receiving members for flexing in response to said load, said flexure means comprising first and second flexures spaced apart in parallel relationship relative to each other and secured between said base and load receiving members to form a parallelogram therewith,   sensing means for producing a signal in response to stresses sensed thereby due to the magnitude of said load, and   adjustment means for selectively redistributing said stresses to provide at least substantially the same said signal independent of the location of said load on said load receiving member, said adjustment means comprising first adjustment means interconnected between said first flexure and said base member for selectively moving said first flexure relative to said base member to redistribute stresses in said first flexure and second adjustment means interconnected between said second flexure and each of said base and load receiving members for selectively redistributing stresses in said second flexure.